The present invention relates to a bearing system for a sand container to be vibrated on a vibrating table in a lost foam casting apparatus. In particular, the invention relates to a bearing system for a sand container to be vibrated on a vibrating table in a lost foam casting installation, the system comprising a plurality of bearing members projecting upwardly from the vibrating table, each bearing member having an upwardly tapered frusto-conical surface, and a corresponding plurality of frusto-conical bearing sockets on the bottom of the container. A bearing system of this type is described in U.S. Pat. No. 4,859,070.
As is known, the lost foam casting technique is a foundry technique based essentially on the production of a polystyrene (or similar material) pattern which reproduces the characteristics of the piece to be made. The pattern is introduced into a container filled with sand which, by means of vibration, is distributed and compacted in such a way as intimately to closely reproduce the shape of the pattern. Subsequently, hot casting material (typically molten metal) is poured into the space occupied by the pattern. The casting material dissolves the pattern and occupies the space previously occupied thereby within the sand. The final result is a casting, and thus a workpiece, the shape of which copies exactly the shape of the pattern.
Conventional systems for compacting sand involve either vibration means generating a vertical movement (which due to the shape of the coupling surfaces between the vibrating table and the container are in part transformed into horizontal movement), or vibrating means which generate a rotary motion about a vertical axis. Both systems have been found to have serious limitations of use due to the fact that the ever more complex shape of the patterns to be invested by the sand has lead to the need for an increase in the vibrational stresses.
Conventional vibration systems impose accelerations of several g (3-4) onto a unit the overall weight of which, including the container full of sand and the vibrating table, is about 2000-2500 kg. In these conditions, with casting of particularly complex shapes, the time necessary for vibration to fill the internal cavities of the foam pattern can be 2-3 minutes; extending the vibration time considerably increases the risk of deformation of the surfaces of the polystyrene patterns.
One object of the present invention is to provide a container bearing system adapted to operate correctly when the vibrating table has very much higher accelerations imparted to it, for example of the order of 10-15 g, for the purpose of reducing the vibration times and avoiding the risk that the surfaces of the pattern become deformed, and to improve the compaction of the sand and therefore the efficiency of the installation.
Another object of the invention is to reduce the wear on the bottom of the container and the associated expenses inherent in the maintenance of containers, as well as to extend the useful life of the containers themselves, especially when these are subject to high operating accelerations.
A particular object of the invention is to provide a system comprised of bearing members able to withstand shock caused by the bottom of the container without breaking.
A further object of the invention is to prevent premature wear of the bearing surfaces between the bottom of the container and the vibrating table.